1. Field of the Invention
The invention relates generally to the field of communications. More particularly the invention relates to spread-spectrum communications.
2. Discussion of the Related Art
Conventional radio-frequency (RF) digital-data transmission schemes are in general highly susceptible to errors caused by multipath propagation and other interference conditions. Historically, direct-sequence spread-spectrum techniques have offered reasonably good immunity against long-path (e.g. outdoor) types of reflections, where the time distribution (dispersion) of the arrival of the individual successive signal reflections is generally greater than the effective chipping period (inverse of the spread-spectrum chipping rate) of the transmitted signal.
Outdoor environments often exhibit delay-dispersion profiles in the range of 3-100 μs (typically ˜25 μs RMS maximum [for hilly terrain]) and thus are generally addressed with signal spreading rates of ˜1 Mchips/sec (especially when used with time-offset quadrature modulation schemes such as OQPSK); these are in fact very close to the existing parameters of the IS-95 CDMA cellular-telephone system in wide use throughout the U.S. and in many other countries.
For the indoor propagation case, however, the dispersion times are much shorter—typical figures range from 10 to 250 ns, with a median RMS dispersion value of 50 ns. The longer values would imply a minimum spreading rate of ˜4 Mchips/sec, while the shorter (worst-case) values call for spreading rates of about 100 Mchips/sec (and, thus, at least 100 MHz of signal bandwidth for standard direct-sequence (DS) signals using conventional binary phase-shift keying (BPSK) or frequency-shift keying (FSK) modulation. For OQPSK schemes as mentioned above, the minimum required bandwidth is halved but is still nevertheless very unwieldy in crowded RF bands.
Obviously, this bandwidth is not reasonably obtainable in any of the available Industrial, Scientific, and Medical (ISM) bands currently allocated for spread-spectrum transmission in the U.S. below 5 GHz (and would require the whole available 100-MHz band for ISM and Unlicensed National Information Infrastructure [U-NII] applications above 5 GHz), so other techniques must be applied to overcome the multipath problem for indoor wireless links. One prevalent option is to employ frequency hopping, so that via the periodic carrier-frequency changes, the signal will hop to frequencies which do not exhibit multipath nulls (destructive interferences) from the transmitter at the desired receiving points. In general, the total received RF energy of many of these data bursts (hops) will be cancelled by the nulls (and thus produce bad data packets), but generally a majority will be of satisfactory quality to provide reasonably effective link operation.
However, either complex (and delay-inducing) interleaving and/or error-correction coding algorithms (e.g., Reed-Solomon) must be introduced into the link, or numerous packet retransmissions will be required to successfully transport the data payload. In either case, significant levels of latency and concurrent link-rate limitations will inevitably result.
Some methodologies have even been developed to build “hopping tables” of useable (low-error) frequencies in the system-control software, and thereby avoid the frequencies with propagation nulls, but in general as the signal-transmission environment changes with movements of equipment, personnel, and RF interference sources, the set of “bad” frequencies will need to be constantly updated; even so; statistically and practically, some bad packets will nevertheless always be received. Furthermore, the use of “intelligent” hopping schemes which on average avoid certain hopping channels in a coordinated fashion have been historically disallowed by the Federal Communications Commission (FCC) on the ISM bands, since the average channel occupancy would be nonuniform and thereby skew the normal long-term random signal-frequency statistics intended for ISM-band frequency-hopping system operation, resulting in a statistical increase in interference to other users (although via rules changes by the FCC in the last few months, this prohibition has been somewhat relaxed). Overall, however, this scheme, although usually workable for fixed devices, generally fails in mobile applications or when the RF environment is dynamic, since the positions of the multipath nulls (and thus the sets of “bad” channels) are constantly changing.
Heretofore, the requirements of a more robust scheme (fewer data errors) which will function effectively even in severe multipath environments (e.g., highly RF-reflective areas) and yet avoid the introduction of either extremely complex error-correction hardware with substantial latency (delay) into the transmission process and/or the requirement for frequent retransmissions, plus offering a solution to the issue of link latencies (which can be particularly significant in high-speed control applications where the delays can cause loop-stability problems for the RF-in-the-loop systems referred to above) have not been fully met. For operation in the United States, the scheme must additionally comply with FCC Part 15 regulations for the ISM and U-NII bands by guaranteeing adequately random spectral characteristics of its transmissions at all times.
Still another essential aspect of modern RF telemetry systems is that of efficient power utilization. It is desirable to operate many distributed devices, including sensors, alarm systems, RFID tags, and the like from low-cost, compact battery sources for maintenance-free intervals of 1 to about 5 years (or even longer). It is therefore highly desirable to provide a system RF telemetry protocol which achieves reliable data transmission with an absolute minimum of remote-device power consumption.
Yet another critical need in many systems is to simultaneously operate a large number of RF devices (such as tags, sensors, and the like) in a proximal area without significant statistical levels of mutual interference; in common parlance, this is the familiar multiple-access problem which is handled by well known frequency-, time-, or code-division multiplexing or multiple-access techniques (typically referred to as FDMA, TDMA, and CDMA, respectively). However, these methods have not hitherto been simultaneously employed in a programmably or adaptively coupled or coordinated fashion to provide a useful increase in the permissible number of devices operable in a given area for a specific amount of mutual interference.
Still another need is for an RF signal-transmission technique which even in the presence of multipath and multi-user interference can support an accurate radiolocation function where the respective locations of the RF devices can be readily detected, such as for equipment, container, and personnel tracking.
Another key need is for an RF signaling protocol which offers improved transmission security against reception, decoding, or even detection by unauthorized parties.
Finally, a need exists for an RF signaling technique which also provides a high degree of signal programmability and adaptability to rapidly accomplish tradeoffs in the DS code lengths, frequency- and time-hopping patterns, and the interrelationships thereof to effectively address dynamic signal and device-use conditions (e.g., changing multipath and RFI conditions and system functional requirements).
What is needed, then, is a solution that addresses all of these requirements.